The present invention relates to a differential pressure transmitter having a high detection sensitivity by providing a symmetry construction of the transmitter so as to decrease a difference of the transmission coefficient between high and low pressure transmitting paths.
An example of the pressure-receiving structure of a conventional differential pressure transmitter is shown in FIG. 2 which is disclosed in Japanese Patent Laid-Open No. 60-185130(1985), in which a pressure-receiving member is constituted of two parts, body 101 in a high pressure side and body 105 in a low pressure side, and a seal diaphragm 106 is secured on one face of the body 101 and a sensor 144 and an overload protection diaphragm 104 are secured on the other face of it. In FIG. 2, the references numerals identify the element as follows;
101--High pressure side body PA1 102--Sensor assembly PA1 103--Center metallic part PA1 104--Overload protection diaphragm PA1 105--Low pressure side body PA1 106, 107--Seal diaphragm PA1 108, 109--Flange PA1 110, 111--Gasket PA1 112--Bolt PA1 113--Nut PA1 114--Connecting metallic part PA1 115, 116--Isolation chamber PA1 117, 118--Measurement fluid pressure receiving chamber PA1 144--Semiconductor differential pressure sensor
A seal diaphragm 107 is also secured on one face of the body 105 in the low pressure side, on the other face of it, the bodies 101 and 105 are so put on one another as to push the overload protection diaphragm 104 of the body 101, and the outer peripheries of them are welded.
First and second pressure-receiving chambers 117, 118 formed by seal diaphragms 106, 107 for receiving the pressures of first and second fluids are constituted on both sides of a body constituted of the bodies 101 and 105. First and second isolation chambers 115, 116 are formed by the body 101, sensor assembly 102, the overload protection diaphragm 104, and the body 105.
Moreover, the following paths are constituted: a pressure transmitting path for connecting the first pressure-receiving chamber 117 to the first isolation chamber 115, a pressure transmitting path for connecting the first isolation chamber 115 to a semiconductor differential pressure sensor, a pressure transmitting path for connecting the second pressure-receiving chamber 118 to the second isolation chamber 116, and a pressure transmitting path for connecting the second isolation chamber 116 to the semiconductor differential pressure sensor.
The first pressure-receiving chamber 117, first isolation chamber 115, semiconductor differential pressure sensor, and pressure transmitting paths for connecting them are filled with sealing liquid. Similarly, the second pressure-receiving chamber 118, second isolation chamber 116, semiconductor differential pressure sensor, and pressure transmitting paths for connecting them are also filled with sealing liquid, so that pressure is transmitted to the semiconductor differential pressure sensor.
In such a conventional differential pressure transmitter, transmitting coefficients of the respective pressure transmitting paths from the pressure-receiving chambers 117, 118 to the differential pressure sensor in high and low pressure sides were not the same because of it's unsymmetrical construction. Therefore, the transient pressure transmitted from the pressure-receiving chamber 117 or 118 is directly transmitted to the differential pressure sensor so as to give a damage to the sensor. Of course, the overload protection diaphragm 104 is provided in order to protect the sensor, but the difference between the transmitting coefficients of pressure transmitting paths connected to the overload protection diaphragm 104 in both high and low pressure sides make it impossible to perfectly protect the sensor. Therefore, it is difficult to obtain a high sensitivity differential pressure sensor.
Furthermore, U.S. Pat. No. 4,135,407 discloses a differential pressure sensor having a symmetrical construction in which a overload protection apparatus is disposed in a center position of the two pressure isolation diaphragms in high and low pressure sides.
But, the overload protection apparatus is constructed with bellows means and therefore, it is not suitable for providing the differential pressure sensor having a very high sensitivity.